Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points.

In the report, published in the Proceedings of the National Academy of Sciences, research conducted in New Zealand showed that teens who started smoking marijuana before age 18 and were diagnosed as being addicted to cannabis by age 38 experienced an IQ drop in early adulthood. But users who began smoking after age 18 — even if they used heavily — did not show a significant decline.

“The effect of cannabis on IQ is really confined to adolescent users,” says lead author Madeline Meier, a postdoctoral researcher at Duke University. “Our hypothesis is that we see this IQ decline in adolescence because the adolescent brain is still developing and if you introduce cannabis, it might interrupt these critical developmental processes.”

The authors followed 1,037 children born in the town of Dunedin, New Zealand, in 1972 and 1973 — virtually every child. They defined adolescent use as at least weekly use before turning 18. In looking at the relationship between marijuana use and IQ, they controlled for factors like years of education, schizophrenia and use of alcohol or other drugs that might also have an effect on IQ. While education weakened the relationship, it still did not eliminate it.

Researchers also had family members and friends of the participants confidentially rate them on attention and memory skills and those who had lost IQ points showed problems in these areas. Meier notes that an eight-point decline in IQ for someone with average intelligence (an IQ score of 100, the 50th percentile) would move that person down to the 29th percentile. “It’s fairly substantial but it does depend on where you start out,” she says.

“I think this is the cleanest study I’ve ever read” exploring the long term effects of marijuana use, Dr. Nora Volkow, director of the National Institute on Drug Abuse, told the Associated Press.

“The overall implication is that when you’re talking about marijuana, you have to take into account age of onset of use and dealing with developing, growing brains,” says Meier.

Not all experts agree, however. “Scientifically, these are extremely preliminary findings,” cautions Carl Hart, associate professor of psychology at Columbia University, who has studied the cognitive effects of marijuana in humans in the lab and was not associated with the research. [Disclosure: Hart and I are working on a book project together.]

Hart notes that because only 38 people in the study — around 8% of those who ever tried marijuana — used it heavily enough to get diagnosed with dependence during several follow-up periods, he is skeptical about how generalizable the results are. He says that in his studies of people who smoke at least three times a week, “when you compare these people’s scores to a normative database on a wide range of domains including executive function, memory and inhibitory control, they score dead smack in the middle, in the 50th percentile.”

He explains: “They are normal when not intoxicated. We test them when they are not intoxicated and when they are intoxicated. When they are intoxicated, there is some slowing of certain cognitive acts, but their accuracy doesn’t not change.” The New Zealand study, for example, did not identify whether the participants were employed or whether they were able to function in their families, which would be an important indicator of whether the drop in IQ has any real-world impact.

There are also other factors — such as child abuse or other trauma — that might lead people to seek escape in heavy marijuana use and could also affect brain function. Meier and her colleagues did not examine these factors but say it’s possible that such elements could explain the results better than marijuana itself.

If the link is real, the effects on cognition could be dramatic. But intelligence and cognition is affected by a plethora of other factors, including genetic, social and environmental influences that may supersede any influence from drug use. Despite the fact that the average marijuana user starts at age 17 in the U.S and nearly 7% of high school seniors currently smoke pot every day, IQ scores have risen tremendously over time in all developed countries in recent years. Most of those same countries also experienced a massive increase in marijuana use between the 1950s and today.